Patent application title: ADJUSTABLE IMPEDANCE MATCHING CIRCUIT

Abstract:

An implantable medical device with a medical module, an antenna, a
transceiver and an impedance match circuit. The transceiver is
operatively coupled to the antenna and the medical module and facilitates
wireless transmission of data between the medical module and an external
device. The impedance match circuit is operatively coupled between the
transceiver and the antenna and has a plurality of predetermined
selectable configurations, each providing a particular impedance matching
characteristic.

Claims:

1. An implantable medical device, comprising:a medical module;an antenna;a
transceiver operatively coupled to said antenna and said medical module
and facilitating wireless transmission of data between said medical
module and an external device; anda impedance match circuit operatively
coupled between said transceiver and said antenna, said impedance match
circuit having a plurality of predetermined selectable configurations,
each individual one of the plurality of configurations providing a
particular impedance matching characteristic.

2. The implantable medical device of claim 1 wherein a first configuration
and a second configuration of said plurality of predetermined selectable
configurations correspond to operating said antenna in air and implanted
in a patient, respectively.

3. The implantable medical device of claim 2 further comprising a memory
module operatively coupled to said impedance match circuit, said memory
module storing said first configuration and said second configuration.

4. The implantable medical device of claim 3 wherein said first
configuration and said second configuration are selectable by a command
generated external to said implantable medical device.

5. The implantable medical device of claim 4 wherein said antenna is
configured to receive a wireless signal, wherein the transceiver is
configured to identify said command contained within said wireless signal
and wherein one of said first configuration and said second configuration
is selected based on said command.

6. The implantable medical device of claim 3 wherein said memory module
stores a plurality of codes, each one of said plurality of codes being
representative of one of said first configuration and said second
configuration.

7. The implantable medical device of claim 6 wherein one of said first
configuration and said second configuration is selected based, at least
in part, on a selection of one of said plurality of codes from a command
generated external to said implantable medical device.

8. The implantable medical device of claim 7 wherein said transceiver is
configured to receive a selection of said one of said plurality of codes
from a wireless signal.

9. A method for matching impedance with an impedance match circuit
operatively coupled between an antenna and a transceiver of an
implantable medical device, said impedance match circuit having a
plurality of predetermined configurations, each individual one of the
plurality of configurations providing a particular impedance matching
characteristic, comprising the steps of:selecting a first one of said
plurality of predetermined configurations corresponding to a first
environment in which said antenna; thenmoving said antenna from said
first environment to a second environment; andselecting a second one of
said plurality of predetermined configurations corresponding to said
second environment.

10. The method of claim 9 wherein said first one of said plurality of
predetermined configurations and said second one of said plurality of
predetermined configurations correspond to operating said antenna in air
and implanted in a patient, respectively.

11. The method of claim 9 wherein said implantable medical device further
comprises a memory module, and further comprising the step, before said
selecting a first one of said plurality of predetermined configurations
step, of storing said first one and said second one of said plurality of
predetermined configurations in said memory module.

12. A computer readable medium having computer executable instructions for
performing a method for matching impedance with an impedance match
circuit operatively coupled between an antenna and a transceiver of an
implantable medical device, said impedance match circuit having a
plurality of predetermined configurations, each individual one of the
plurality of configurations providing a particular impedance matching
characteristic, comprising:selecting a first one of said plurality of
predetermined configurations corresponding to a first environment in
which said antenna; thenmoving said antenna from said first environment
to a second environment; andselecting a second one of said plurality of
predetermined configurations corresponding to said second environment.

13. The computer readable medium of claim 12 wherein said first one of
said plurality of predetermined configurations and said second one of
said plurality of predetermined configurations correspond to operating
said antenna in air and implanted in a patient, respectively.

14. An implantable medical device, comprising:a medical module;an
antenna;transceiver means operatively coupled to said antenna and said
medical module for facilitating wireless transmission of data between
said medical module and an external device; andimpedance match means
operatively coupled between said transceiver means and said antenna, said
impedance match means for providing a plurality of predetermined
selectable configurations, each individual one of the plurality of
configurations providing a particular impedance matching characteristic.

15. The implantable medical device of claim 14 wherein a first
configuration and a second configuration of said plurality of
predetermined selectable configurations correspond to operating said
antenna in air and implanted in a patient, respectively.

16. The implantable medical device of claim 15 further comprising memory
means operatively coupled to said impedance match means, said memory
means for storing said first configuration and said second configuration.

Description:

[0002]This disclosure relates to telemetry communication with implantable
medical devices and, in particular, impedance matching to adapt such
telemetry communications to the environment in which the implantable
medical device is operating.

BACKGROUND

[0003]The use of wireless communications in implantable medical devices is
well known in the art. Using both inductive and radio frequency
communications, data and commands may be transmitted to an implantable
medical device and telemetry data may be received from the implantable
medical device. In a radio frequency application, the implantable medical
device may utilize a relatively small, space-efficient antenna coupled to
an internal transceiver to establish a communication link with an antenna
of an external device positioned in proximity of the internal antenna.

[0004]The effective range and rate of radio frequency communication may
depend in part on a degree to which the impedance of the antenna of the
external device matches the impedance of the antenna of the internal
device. The closer the impedance match, the clearer the signal between
the two antennas may be and the greater the rate the communication may
be. Beyond the impact of variance in the componentry utilized in wireless
communications, the environment in which the implantable medical device
operates may have an impact on the perceived impedance of the internal
antenna.

[0005]For instance, U.S. Pat. No. 7,409,245, Larson et al., discloses a
variable antenna matching network for an implantable antenna. Changes in
the patient's body position, weight, composition or other factors may
change the antenna efficiency and hinder communication. The disclosed
circuit may automatically adjust a matching network for an implanted
transceiver to dynamically maximize transmission and reception by
controlling the selected value of a plurality of capacitors, inductors
and resistors.

[0006]However, because of the premium placed on making implantable medical
devices relatively small, many internal antennas are not tunable. As a
result, manufacturers of implantable medical devices have traditionally
made a compromise between maximizing wireless communication efficiency
and range and keeping the internal volume of the implantable medical
device small.

SUMMARY

[0007]Thus, while real-time tunable impedance matching circuits have been
utilized to improve telemetry communication with implantable medical
devices, such circuits typically expend a significant amount of
implantable medical device resources including increasing the internal
volume of the implantable medical device, expending computing power of
the implantable medical device, and reducing the service life of a power
source (for instance, a battery) of the implantable medical device.

[0008]In an embodiment, an impedance match circuit is provided for an
implantable medical device which, instead of being tunable in real-time,
has a plurality of predetermined selectable configurations. In one
example, one of the predetermined configurations may correspond to the
antenna operating in air, while another of the predetermined
configurations may correspond to the antenna operating after implantation
in a patient. Alternative configurations may also be provided for
alternative circumstances. By establishing predetermined configurations,
impedance matching may be provided which is smaller, simpler, and less
wasteful of implantable medical device resources such as computing power
and system power, among other benefits.

[0009]In an embodiment, an implantable medical device comprises a medical
module, an antenna, a transceiver and an impedance match circuit. The
transceiver is operatively coupled to the antenna and the medical module
and facilitates wireless transmission of data between the medical module
and an external device. The impedance match circuit is operatively
coupled between the transceiver and the antenna and has a plurality of
predetermined selectable configurations, each providing a particular
impedance matching characteristic.

[0010]In an embodiment, a first configuration and a second configuration
of the plurality of predetermined selectable configurations correspond to
operating the antenna in air and implanted in a patient, respectively.

[0011]In an embodiment, the implantable medical device further comprises a
memory module operatively coupled to the impedance match circuit, the
memory module storing the first configuration and the second
configuration.

[0012]In an embodiment, the first configuration and the second
configuration are selectable by a command generated external to the
implantable medical device.

[0013]In an embodiment, the antenna is configured to receive a wireless
signal, wherein the transceiver is configured to identify the command
contained within the wireless signal and wherein one of the first
configuration and the second configuration is selected based on the
command.

[0014]In an embodiment, the memory module stores a plurality of codes,
each one of the plurality of codes being representative of one of the
first configuration and the second configuration.

[0015]In an embodiment, one of the first configuration and the second
configuration is selected based, at least in part, on a selection of one
of the plurality of codes from a command generated external to the
implantable medical device.

[0016]In an embodiment, the transceiver is configured to receive a
selection of the one of the plurality of codes from a wireless signal.

[0017]In an embodiment, a method is disclosed for matching impedance with
an impedance match circuit operatively coupled between an antenna and a
transceiver of an implantable medical device, the impedance match circuit
having a plurality of predetermined configurations, each individual one
of the plurality of configurations providing a particular impedance
matching characteristic. A first one of the plurality of predetermined
configurations corresponding to a first environment in which the antenna
is selected. Then the antenna is moved from the first environment to a
second environment. A second one of the plurality of predetermined
configurations corresponding to the second environment is selected.

[0018]In an embodiment, the implantable medical device further comprises a
memory module, and the method further comprises the step, before the
selecting a first one of the plurality of predetermined configurations
step, of storing the first one and the second one of the plurality of
predetermined configurations in the memory module.

[0019]In an embodiment, a computer readable medium is disclosed having
computer executable instructions for performing a method for matching
impedance with an impedance match circuit operatively coupled between an
antenna and a transceiver of an implantable medical device, the impedance
match circuit having a plurality of predetermined configurations, each
individual one of the plurality of configurations providing a particular
impedance matching characteristic. A first one of the plurality of
predetermined configurations corresponding to a first environment in
which the antenna is selected. Then the antenna is moved from the first
environment to a second environment. A second one of the plurality of
predetermined configurations corresponding to the second environment is
selected.

DRAWINGS

[0020]FIG. 1 is a schematic diagram illustrating one embodiment of a
communication system for communicating medical data between an
implantable medical device (implantable medical device) and an external
unit;

[0021]FIG. 2 is block diagram illustrating one embodiment of a portion of
an implantable medical device circuit;

[0022]FIG. 3 is an electrical schematic diagram illustrating one
embodiment of a portion of an implantable medical device circuit;

[0023]FIG. 4 is an electrical schematic diagram illustrating one
embodiment of an antenna impedance matching circuit including a
selectable topology;

[0024]FIG. 5 is one embodiment of a selection chart for selecting an
antenna impedance matching circuit topology;

[0025]FIG. 6A is an electrical schematic diagram illustrating one
embodiment of the antenna impedance matching circuit in a first topology;

[0026]FIG. 6B is an electrical schematic diagram illustrating one
embodiment of the antenna impedance matching circuit in a second
topology;

[0027]FIG. 6c is an electrical schematic diagram illustrating one
embodiment of the antenna impedance matching circuit in a third topology;
and

[0028]FIG. 6D is an electrical schematic diagram illustrating one
embodiment of the antenna impedance matching circuit in a fourth
topology.

[0030]FIG. 1 is a schematic diagram illustrating one embodiment of a
communication system 100 for communication between an implantable medical
device 108, which includes lead 112 and antenna 110, and external unit
104. In one embodiment, implantable medical device 108 is an implantable
cardioverter-defibrillator, but the embodiments of the present invention
are equally applicable to many types of medical devices, including both
implantable medical devices and external medical devices. For example,
implantable medical device 108 may provide electrical stimulation therapy
(e.g., a combination at least one of pacing, defibrillation,
cardioversion or cardiac resynchronization therapy). In other instances,
implantable medical device 108 may provide electrical stimulation therapy
to other regions of the body, e.g., a spine, brain, or the like. In yet
another example, implantable medical device 108 may provide drug delivery
therapy or other therapies in addition to or instead of electrical
stimulation therapy.

[0031]In addition to or instead of providing therapy, implantable medical
device 108 may be capable of sensing physiological events of the heart of
patient 102 via electrodes of lead 112. Implantable medical device 108
may also sense one or more physiological or biological conditions of
other regions of patient 102 via electrodes of lead 112 or other sensors
on lead 112, within implantable medical device 108 or separate
stand-alone sensors. Antenna 110 is used to communicate with external
unit 104 and may be any suitable device capable of sending or receiving
electromagnetic waves, including for example a surface mounted antenna,
an inductor, or a half-wave strip.

[0032]External unit 104 is a device, such as a programmer or home monitor,
capable of bi-directional communication with implantable medical device
108 via antenna 106.

[0033]External unit 104 includes antenna 106, which may be any suitable
type of radio frequency antenna capable of communicating in the desired
radio frequency frequencies with implantable medical device 108, and may
be located inside or outside of a housing of external unit 104.

[0034]Implantable medical device 108 includes an adjustable impedance
matching circuit for impedance matching antenna 110 to a radio frequency
transceiver within the device can of implantable medical device 108. By
adjusting the impedance matching circuit between different predetermined
impedances, operation of antenna may be increased. Moreover, providing
the capability to adjust between different predetermined impedances
allows better impedance matches to be achieved between antenna 110 and
the transceiver of implantable medical device in different operating
environments, e.g., prior to implantation (in air) and after implantation
(in the body). Additionally, a variety of antennas 110 having different
impedances may by used with a single radio frequency transceiver used in
different implantable medical devices without having to use a separate
impedance matching circuit for each of the variety of antennas.
Therefore, both development times and costs are reduced while improving
communication performance.

[0035]FIG. 2 is block diagram illustrating one embodiment of a portion of
an implantable medical device circuit. The circuit includes antenna 110,
the implantable medical device 108 device can 120, a high voltage
protection circuit 128, an antenna impedance matching circuit 136, and a
radio frequency transceiver 142. Antenna 110 is electrically coupled to
an input of high voltage protection circuit 128 through signal path 122.
Signal path 122 passes through a feed through 124 of device can 120. Feed
through 124 is electrically coupled to device can 120 and a second input
of high voltage transceiver 128 through signal path 126.

[0036]Medical module 111 is coupled to lead 112 and provides sensing
and/or therapy functions consistent with those commonly provided in
implantable medical devices such as pacemakers,
cardioverters/defibrillators, neurological stimulators and drug infusion
devices. In various embodiments, medical module 111 is operatively
coupled to radio frequency transceiver 142 and receives instructions and
transmits data to external unit 104 by way of radio frequency transceiver
142. In certain embodiments, medical module 111 is directly coupled to
radio frequency transceiver 142. In alternative embodiments various
electronics modules commonly known in the art are coupled between radio
frequency transceiver 142 and medical module 111 in order to facilitate
communication between radio frequency transceiver 142 and medical module
111, including controllers and data storage.

[0037]A first output of high voltage protection circuit 128 is
electrically coupled to a first input of antenna impedance matching
circuit 136 through signal path 130. A second output of high voltage
protection circuit 128 is electrically coupled to a second input of
antenna impedance matching circuit 136 and to a common or ground 134
through signal path 132. A first output of antenna impedance matching
circuit 136 is electrically coupled to a first input of radio frequency
transceiver 142 through signal path or node 138. A second output of
antenna impedance matching circuit 136 is electrically coupled to a
second input of radio frequency transceiver 142 and to common or ground
134 through signal path or node 140.

[0038]Antenna 110 receives radio frequency signals from external unit 104
(FIG. 1) and transmits radio frequency signals to external unit 104. Feed
through 124 is hermetically sealed such that circuits within device can
120 are protected when implanted within a patient. In one embodiment,
device can 120 includes titanium or other suitable material. High voltage
protection circuit 128, antenna impedance matching circuit 136, radio
frequency transceiver 142, and other implantable medical device 108
circuitry (not shown) is enclosed within device can 120.

[0039]High voltage protection circuit 128 protects implantable medical
device 108 circuitry from high voltages on signal path 122 and/or 126. In
one embodiment, high voltage protection circuit 128 includes a radio
frequency limiter to limit the amount of radio frequency energy that is
allowed to pass through to antenna impedance matching circuit 136. As
such, protection circuit 128 protects implantable medical device 108
circuitry from radiation emitted from, for example, two-way radios,
magnetic resonance imaging (MRI) machines, or other radiation to which a
patient may be exposed.

[0040]Antenna impedance matching circuit 136 matches the impedance of
antenna 110 to the impedance of radio frequency transceiver 142. In one
embodiment, radio frequency transceiver 142 has a receiver input
impedance of 50 ohms and a transmitter output impedance of 50 ohms
between nodes 138 and 140, and antenna 110 has an impedance less than 50
ohms. Antenna impedance matching circuit 136 matches the lower impedance
of antenna 110 to the higher impedance of radio frequency transceiver
142. Antenna impedance matching circuit 136 can be adjusted to match the
impedance of antenna 110 to the impedance (or as close to as possible)
radio frequency transceiver 142 in different operating environments. For
example, antenna impedance matching circuit 136 may have one
predetermined configuration corresponding to the antenna operating in air
and another predetermined configuration corresponding to the antenna
operating after implantation in a patient. Antenna impedance matching
circuit 136 can also be adjusted to match different antennas 110 having
different impedances to radio frequency transceiver 142 in addition to or
instead of adjusting the impedance in different operating environments.
In this way, multiple implantable medical devices 108 having different
designs and antenna designs having different impedances can use the same
antenna impedance matching circuit 136 and radio frequency transceiver
142. Therefore, a unique antenna impedance matching circuit 136 and/or
radio frequency transceiver is not needed for each individual implantable
medical device 108 design or antenna design.

[0041]In various embodiments, impedance matching circuit 136 is coupled to
memory module 135. In such embodiments, codes to configure impedance
matching circuit 136 may be stored in memory module 135 and transmitted
to impedance matching circuit 136 at appropriate times. The appropriate
times may be determined on the basis of conditions sensed by medical
module 111. For example, antenna impedance matching circuit 136 may be
configured to switch from the predetermined configuration corresponding
to the antenna operating in air to the predetermined configuration
corresponding to the antenna operating upon detecting a cardiac signal
via one of the leads. Detection of a cardiac signal may indicate that
implantable medical device 108 is implanted within patient 112. In
alternative embodiments, commands may be transmitted from external unit
104 by way of antenna 110 and identified by radio frequency transceiver
142 or a coupled controller which are configured to identify the command
and cause the code to be transmitted from memory module 135 to impedance
matching circuit 136 to adjust the impedance. The command may, for
example, be transmitted by a programmer after implantation.

[0042]On the basis of the codes transmitted to impedance matching circuit
136, impedance matching circuit 136 may be reconfigured in a manner
described in detail below. In certain embodiments, memory module 135 may
also be coupled to radio frequency transceiver 142 and may be loaded with
newly transmitted codes. In particular, in various embodiments, memory
module 135 may be pre-stored with derived codes corresponding to
predetermined selectable configurations. In certain embodiments, such
configurations relate to implantable medical device 108 operating outside
of patient 102, and implantable medical device operating implanted within
patient 102. In order to derive such codes, antenna 110 may be replaced
by test loads simulating the various conditions in which implantable
medical device 108 may operate. In alternative embodiments, the
predetermined selectable configurations relate to other conditions in
which implantable medical device 108 may be operating, and may be
obtained either on the basis of simulated test loads or through
configurations predetermined in ways related to devices of the same type
as implantable medical device 108 but not necessarily to implantable
medical device 108 individually.

[0043]In one embodiment, antenna impedance matching circuit 136 includes
one inductor and one variable capacitor, such as a veractor or switched
capacitor, to impedance match antenna 110 to radio frequency transceiver
142. In various alternative embodiments, antenna impedance matching
circuit 136 includes various combinations of inductors and variable
capacitors in a selectable configuration. Such alternative embodiments
may include embodiments with no inductors which rely only on capacitors,
and embodiments with no capacitors which rely only on inductors.

[0045]In one embodiment, the output power of radio frequency transceiver
142 is checked periodically (or often enough to be nearly continuous).
The output power is greatest when antenna 110 is properly matched to
radio frequency transceiver 142. In other embodiments, the received
signal strength is maximized to properly impedance match antenna 110 to
radio frequency transceiver 142 maximizing the output power or received
signal strength to compensate for antenna feedpoint impedance variations
as can occur (i.e. a hand-held instrument or an implant prior and after
implant).

[0046]FIG. 3 is an electrical schematic diagram illustrating one
embodiment of a portion of an implantable medical device circuit. The
circuit includes antenna 110, device can 120, capacitor 128 and antenna
impedance matching circuit 136. Antenna impedance matching circuit 136
includes capacitors 166 and 178, inductors 152, 154, and 174, varactors
158 and 170, and resistor 164. Antenna 110 is electrically coupled to one
side of inductor 152 and one side of inductor 154 through signal paths
122 and 130. Signal path 122 passes through a feed through 124 of device
can 120 and becomes signal path 130. Feed through 124 is electrically
coupled to device can 120 and one side of capacitor 128 through signal
path 126. The other side of capacitor 128 is electrically coupled to
common or ground 134 and the other side of inductor 152 through signal
path 132.

[0047]The other side of inductor 154 is electrically coupled to the anode
of varactor 158 through signal path 156. The cathode of varactor 158 is
electrically coupled to one side of resistor 164 and one side of
capacitor 166 through signal path or node 138. The other side of resistor
164 is electrically coupled to a first voltage source (V1) 160 through
signal path 162. The other side of capacitor 166 is electrically coupled
to the cathode of varactor 170 and one side of inductor 174 through
signal path 168. The anode of varactor 170 is electrically coupled to
common or ground 134 through signal path 172. The other side of inductor
174 is electrically coupled to a second voltage source (V2) 180 and one
side of capacitor 178 through signal path 176. The other side of
capacitor 178 is electrically coupled to common or ground 134 through
signal path or node 140.

[0048]Capacitor 128 provides high voltage protection for impedance
matching circuit 136 and the radio frequency transceiver. In other
embodiments, capacitor 128 is replaced with other suitable components for
providing high voltage protection and/or for providing a radio frequency
limiter. In one embodiment, inductors 152, 154, and 174 are off chip, and
resistor 164, capacitors 166 and 178, and varactors 158 and 170 are on
chip. Resistor 164 decouples first voltage source 160 from the radio
frequency signal on signal path 138.

[0049]Varactors 158 and 170 each provide a variable capacitance for
impedance matching antenna 110 to radio frequency transceiver 142, which
is coupled to nodes 138 and 140. First voltage 160 and second voltage 180
are adjusted to adjust the capacitance of varactors 158 and 170. In one
embodiment, first voltage 160 and second voltage 180 are provided by
digital to analog converters (DACs). First voltage 160 and second voltage
180 can be adjusted once at factory calibration, adjusted in response to
a command from external unit 104, and/or automatically adjusted to
impedance match antenna 110 to radio frequency transceiver 142 for
optimal performance.

[0050]FIG. 4 is an electrical schematic diagram illustrating one
embodiment of an antenna impedance matching circuit 220 including a
selectable topology. In one embodiment, antenna impedance matching
circuit 220 is used in place of antenna impedance matching circuit 136
previously described and illustrated with reference to FIG. 2. Antenna
impedance matching circuit 220 includes inductors 224 (L2) and 238 (L1)
and tunable capacitors 228 and 242. In an embodiment, inductor 224 is 18
nanohenrys and inductor 238 is 10 nanohenrys. In an embodiment, tunable
capacitors 228 and 242 have a range from 0.25 picoFarads to 25.0
picoFarads. In various alternative embodiments, the values of inductors
224 and 238 and of tunable capacitors 228 and 242 may vary as needed. In
particular, inductors 224, 238 may be selected based on characteristics
of antenna 110 and transceiver 142, while tunable capacitors 228, 242 may
have a lower value as close to 0.0 Farads as can be achieved and an upper
value based on characteristics of antenna 100 and transceiver 142. In
various embodiments, tunable capacitors 228, 242 are formed on a common
die with

[0051]RFSIG1 signal path or node 222 is electrically coupled to one side
of inductor 224. The other side of inductor 224 is electrically coupled
to one side of tunable capacitor 228 through RFSIG3 signal path or node
226. The other side of tunable capacitor 228 is electrically coupled to a
common or ground 234, one side of tunable capacitor 242, and node 140
through signal path 230. The other side of tunable capacitor 242 is
electrically coupled one side of inductor 238 and node 138 through RFSIG2
signal path or node 240. The other side of inductor 238 is electrically
coupled to RFSIG4 signal path or node 236.

[0052]In one embodiment, inductors 224 and 238 and tunable capacitors 228
and 242 are arranged into a topology based on selection chart 200
previously described and illustrated with reference to FIG. 5. In one
embodiment, nodes 222, 226, 236, and 240 include pads accessible to the
user for coupling antenna 110 and for coupling jumpers between the nodes
for selecting the topology. As such the topologies which are illustrated
in FIGS. 6A-6D are preselected configurations of impedance match circuit
136 of one embodiment. However, alternative jumper configurations may be
selected between and among nodes 222, 226, 236 and 240 to create
alternative topologies with alternative characteristics. Such alternative
characteristics may be plotted on chart 200 of FIG. 5 and incorporated
into decisions related to configuring impedance matching circuit 136 for
various antennas 110 and transceivers 142, and the various embodiments in
which antenna 110 operates.

[0053]Tunable capacitors 228 and 242 are adjusted in the selected topology
to impedance match antenna 110 to the radio frequency transceiver, which
is coupled to nodes 138 and 140. In one embodiment, each tunable
capacitor 228 and 242 is a binary weighted switched capacitor bank, a
linear weighted switched capacitor bank, or another suitable tunable
capacitor that can be tuned digitally. Tunable capacitors 228 and 242 are
set using a search, sweep, or other suitable method to match the
impedance of antenna 110 to the radio frequency transceiver in the
selected topology.

[0054]FIG. 5 is one embodiment of a selection chart 200 for selecting an
antenna impedance matching circuit topology. Selection chart 200 is used
in combination with an antenna impedance matching circuit 220 that will
be described below with reference to FIG. 4. Selection chart 200 includes
reactance (Xs) in ohms on x-axis 202 and resistance (Rs) in ohms on
y-axis 204. In this embodiment, the antenna resistance is between 5 ohms
and 50 ohms, and the antenna reactance is between -60 ohms and 0 ohms. A
first topology for impedance values within the range indicated at 206 is
L2+PI(L1), where L1 is a first inductor and L2 is a second inductor. The
first topology is illustrated in FIG. 6A below. A second topology for
impedance values within the range indicated at 208 is PI(L1) and is
illustrated in FIG. 6B below. A third topology for impedance values
within the range indicated at 210 is PI(L2) and is illustrated in FIG. 6c
below. A fourth topology for impedance values within the range indicated
at 212 is PI(L1//L2) and is illustrated in FIG. 6D below.

[0055]For example, in one embodiment for an antenna impedance of 20-j40
ohms, the first topology L2+PI(L1) is selected. For an antenna impedance
of 30-j5 ohms, the second topology PI(L1) is selected. For an antenna
impedance of 40-j15 ohms, the third topology PI(L2) is selected. For an
antenna impedance of 5-j5 ohms, the fourth topology PI(L1//L2) is
selected.

[0056]Antenna impedance matching circuit 220 is configured in one of a
first, second, third, and fourth topology based on selection chart 200
previously described and illustrated with reference to FIG. 4. The
following FIGS. 6A-6D illustrate the first topology (i.e., L2+PI(L1)),
the second topology (i.e., PI(L1)), the third topology (i.e., PI(L2)),
and the fourth topology (i.e., PI(L1//L2)).

[0057]FIG. 6A is an electrical schematic diagram illustrating one
embodiment of antenna impedance matching circuit 220 in the first
topology (i.e., L2+PI(L1)). In this embodiment, antenna 110 is
electrically coupled to RFSIG1 node 222 through signal path 122. RFSIG3
node 226 is electrically coupled to RFSIG4 node 236 through signal path
270. Tunable capacitors 228 and 242 are then tuned to match the impedance
of antenna 110 to the radio frequency transceiver.

[0058]FIG. 6B is an electrical schematic diagram illustrating one
embodiment of antenna impedance matching circuit 220 in the second
topology (i.e., PI(L1)). In this embodiment, antenna 110 is electrically
coupled to RFSIG4 node 236 through signal path 122. RFSIG4 node 236 is
electrically coupled to RFSIG3 node 226 through signal path 272. Tunable
capacitors 228 and 242 are then tuned to match the impedance of antenna
110 to the radio frequency transceiver.

[0059]FIG. 6c is an electrical schematic diagram illustrating one
embodiment of antenna impedance matching circuit 220 in the third
topology (i.e., PI(L2)). In this embodiment, antenna 110 is electrically
coupled to RFSIG3 node 226 through signal path 122. RFSIG1 node 222 is
electrically coupled to RFSIG2 node 240 through signal path 274. Tunable
capacitors 228 and 242 are then tuned to match the impedance of antenna
110 to the radio frequency transceiver.

[0060]FIG. 6D is an electrical schematic diagram illustrating one
embodiment of antenna impedance matching circuit 220 in the fourth
topology (i.e., PI(L1//L2)). In this embodiment, antenna 110 is
electrically coupled to RFSIG4 node 236. RFSIG4 node 236 is electrically
coupled to RFSIG3 node 226 through signal path 276. RFSIG1 node 222 is
electrically coupled to RFSIG2 node 240 through signal path 278. Tunable
capacitors 228 and 242 are then tuned to match the impedance of antenna
110 to the radio frequency transceiver.

[0061]Embodiments provide an adjustable impedance matching circuit for
matching the impedance of an implantable medical device telemetry antenna
to a radio frequency transceiver of the implantable medical device. By
adjusting the impedance matching circuit, the impedance of the antenna
may be matched more closely to the impedance of the radio frequency
transceiver of implantable medical device in different operating
environments. For example, the antenna impedance matching circuit may
have one predetermined configuration corresponding to the antenna
operating in air and another predetermined configuration corresponding to
the antenna operating after implantation in a patient. Alternatively or
additionally, by adjusting the impedance matching circuit based on the
selected telemetry antenna, a variety of telemetry antenna designs can be
used with a single impedance matching circuit and radio frequency
transceiver. Therefore, a different impedance matching circuit need not
be designed for each telemetry antenna design, thereby saving development
time and reducing costs.

[0062]Although the present invention has been described with reference to
preferred embodiments, workers skilled in the art will recognize that
changes can be made in form and detail without departing from the spirit
and scope of the present invention.